Use `__proxy` mechanism for kill/cancelThread. NFC

This was noticed while working on #18376

src/library_pthread.js

diff --git a/src/library_pthread.js b/src/library_pthread.js
index 83d9900..a51b837 100644
--- a/src/library_pthread.js
+++ b/src/library_pthread.js
@@ -29,8 +29,8 @@
 var LibraryPThread = {
   $PThread__postset: 'PThread.init();',
   $PThread__deps: ['_emscripten_thread_init',
-                   '$killThread',
-                   '$cancelThread', '$cleanupThread', '$zeroMemory',
+                   '$zeroMemory',
+                   '$cleanupThread',
                    '$spawnThread',
                    '_emscripten_thread_free_data',
                    'exit',
@@ -267,10 +267,6 @@
           spawnThread(d);
         } else if (cmd === 'cleanupThread') {
           cleanupThread(d['thread']);
-        } else if (cmd === 'killThread') {
-          killThread(d['thread']);
-        } else if (cmd === 'cancelThread') {
-          cancelThread(d['thread']);
         } else if (cmd === 'loaded') {
           worker.loaded = true;
 #if ENVIRONMENT_MAY_BE_NODE
@@ -533,6 +529,9 @@
     // entry point, calls pthread_exit, or acts upon a cancellation.
     // Detached threads are responsible for calling this themselves,
     // otherwise pthread_join is responsible for calling this.
+    // Note: we can't use the 'async' proxy mechanism here since this can be
+    // called from _emscripten_thread_exit after the current thread is been
+    // destroyed (i.e. we can't call back into native code at this point).
     if (!ENVIRONMENT_IS_PTHREAD) cleanupThread(thread);
     else postMessage({ 'cmd': 'cleanupThread', 'thread': thread });
   },
@@ -866,14 +865,15 @@
 #endif
   },
 
-  __pthread_kill_js__deps: ['emscripten_main_browser_thread_id'],
+  __pthread_kill_js__deps: ['emscripten_main_browser_thread_id',
+                            '$cancelThread',
+                            '$killThread'],
+  __pthread_kill_js__proxy: 'async',
   __pthread_kill_js: function(thread, signal) {
     if (signal === {{{ cDefine('SIGCANCEL') }}}) { // Used by pthread_cancel in musl
-      if (!ENVIRONMENT_IS_PTHREAD) cancelThread(thread);
-      else postMessage({ 'cmd': 'cancelThread', 'thread': thread });
+      cancelThread(thread);
     } else {
-      if (!ENVIRONMENT_IS_PTHREAD) killThread(thread);
-      else postMessage({ 'cmd': 'killThread', 'thread': thread });
+      killThread(thread);
     }
     return 0;
   },

test/other/metadce/test_metadce_minimal_pthreads.jssize

diff --git a/test/other/metadce/test_metadce_minimal_pthreads.jssize b/test/other/metadce/test_metadce_minimal_pthreads.jssize
index 84641cd..8af5661 100644
--- a/test/other/metadce/test_metadce_minimal_pthreads.jssize
+++ b/test/other/metadce/test_metadce_minimal_pthreads.jssize
@@ -1 +1 @@
-16064
+15881